We have studied visual spatial attention in humans and monkeys to determine what brain areas are important and what each contributes to this cognitive function. Visual cues which precede visual targets can modulate reaction times to the targets. Cues on the same side as the target (valid cues) are associated with fast reaction times; cues on the opposite side (invalid cues) or weak illumination of the entire visual field (diffuse cues) are correlated with slow reaction times. The cues are hypothesized to control the direction of the subject's attention. Patients with damage to parietal cortex have slowed reaction times to targets which follow diffuse cues or to those cues which draw their attention to the intact visual field. These are both diffuse cues and one invalid cue. We have demonstrated the same deficit in the monkey after surgical removal of cortical area 7. This allows us to study directly a region of cortex in the monkey which is critical for visual spatial attention. Normal human controls or patients with temporal lobe damage do not have these problems. Humans with Alzheimer's dementia are extremely slow in all aspects of this task. Males with idiopathic hypogonadotropic hypogonadism have unusual patterns of responding on our attention task; they are slow in responding to all targets in their right visual field, independent of the preceding cue. They are also slow in responding to targets after diffuse cues. These studies have helped to localize an area of the brain, the inferior parietal lobule, which is critical for visual spatial attention and helped to clarify its contribution to this cognitive process. In addition they have demonstrated a possible endocrine influence on visual behavior.